To deal with the explosive traffic demands on the overloaded cellular network, device-to-device (D2D) offloading is proposed to transmit the traffic originally delivered through cellular network by D2D communications. Current D2D offloading schemes are based on direct transmissions or simple relay-assisted transmissions, which is hard to achieve high throughput. Network coding can effectively solve the problem. However, two problems must be addressed, topology formation and resource allocation. We formulate the topology formation problem as a function on network throughput under transmission pair matching and relay selection. we adopt coalition game theory to jointly match devices to form transmission pairs and select relay to form topology. Moreover, we propose a greedy algorithm to allocate the limited spectrum resource for each transmitter to decrease the interference influence. We evaluate our proposed offloading scheme through extensive evaluations under different network environments. The result shows that our scheme can effectively improve the network throughput and enhance the offloading efficiency.